1. Field of the Invention
The invention relates to the technical field of mixer and, more particularly, to an up-conversion mixing system with high carrier suppression in a radio frequency (RF) transmitter.
2. Description of Related Art
FIG. 1 is a block diagram of a typical RF transmitter. As shown in FIG. 1, the transmitter includes a mixer 110, a power amplifier 120 and an antenna 130. The mixer 110 receives a baseband signal and a local oscillation signal LO in order to shift the baseband signal up to an RF band. Such a frequency conversion is referred to as a direct conversion, which has the features of high integration and low cost and thus is widely used by an RF transmitter.
However, when the status of mixer 110 is not fully differential, or the baseband signal contains a DC offset, a leakage is presented in the output terminal of the mixer 110. The leakage may interfere the spectrum of the RF signal because of the same frequency between the signal LO and the RF signal, then reduce the RF signal quality and affect the performance of the RF transmitter.
To overcome this problem, FIG. 2 is a schematic diagram of a conventional solution for the leakage of a mixer. As shown in FIG. 2, a voltage sampling and hold operation is performed on node 201, 202 to thus obtain DC voltages of the nodes 201, 202. Next, the subtractors 211, 212 subtracts the DC signals from received signals respectively. However, timing control for the sampling circuits increase the complexity and cost of the entire system. In addition, the leakage caused by a mismatch of the mixers 210 cannot be overcome. Therefore, it is desirable to provide an improved mixer to mitigate and/or obviate the aforementioned problems.